FIG. 9 is a partially broken away perspective view showing a configuration of a conventional electric double-layer capacitor of this kind. In FIG. 9, capacitor element 121 is configured by winding anode electrode 122 and cathode electrode 123 in a state where separator 124 is interposed therebetween.
Further, anode electrode 122 and cathode electrode 123 are configured by respectively forming polarizable electrode layers 126 on both sides (the rear side is not shown) of current collector 125 made of metallic foil. Moreover, anode electrode 122 and cathode electrode 123 are respectively connected with anode lead wire 127 and cathode lead wire 128.
Capacitor element 121 as thus configured is impregnated with a driving electrolyte (not shown) and then inserted into bottomed cylindrical case 129, and after rubber-made sealing member 130 having holes through which anode lead wire 127 and cathode lead wire 128 are inserted is arranged on an opening of case 129, a periphery of the opening of case 129 is inwardly drawn and an opening end of case 129 is also curled, to seal the opening.
For the purpose of improvement in efficiency as the electrodes, polarizable electrode layers 126 are formed in layer shape on both sides of current collector 125. In fixing anode lead wire 127 and cathode lead wire 128 for electrical pulling from the electrodes, since contact resistance would be high if the lead wire is fixed from the top of polarizable electrode layer 126, current collector 125 is conventionally exposed by scraping away polarizable electrode layer 126 in positions where anode lead wire 127 and cathode lead wire 128 are fixed or by some other method. Namely, in order to make anode lead wire 127 and cathode lead wire 128 favorably connected with the metallic foil as current collector 125 for reducing a resistance component, polarizable electrode layer 126 in the connected portion is partially removed to expose current collector 125, and the lead wire is then connected thereto.
It is to be noted that as related art document information relevant to the invention of this application, for example, Patent Document 1 and Patent Document 2 are known.
However, in the configuration of the above-mentioned conventional electric double-layer capacitor, especially in cathode polarizable electrode layer 126, when the exposed portion of current collector 125 is opposed to polarizable electrode layer 126 through separator 124 at the time of fixing cathode lead wire 128, overacidity of anions in the electrolyte may exert to degrade or erode the separator.
Further, in the above-mentioned conventional electric double-layer capacitor, at the time of connecting anode lead wire 127 to the anode electrode 122 (the same applies to cathode electrode 123), polarizable electrode layer 126 in the connected portion is partially removed for the purpose of making favorable connection with the metallic foil as current collector 125 to reduce the resistance component. The method of removing polarizable electrode layer 126 is forcibly scraping away the electrode layer by use of a rotating brush or the like, and an area of the electrode layer is removed considerably larger than an area of the connected portion of the lead wire to be connected in view of problems concerning processing accuracy and assembly, and the like. Hence, with the lead wire in the state of being connected to each electrode, on the periphery of the lead wire, polarizable electrode layer 126 does not exist and a portion where current collector 125 is exposed exists.
However, when a requirement for ensuring a range (70° C., 2.7 V), which exceeds a range (60° C., 2.3 V or less) required of the conventional electric double-layer capacitor to ensure, is intended to be met, there is a problem as follows due to occurrence of such an exposed portion of current collector 125.
Namely, when a high voltage is applied to use the capacitor for a long period of time, the driving electrolyte reacts between the exposed portion of current collector 125 of cathode electrode 123 and polarizable electrode layer 126 of anode electrode 122 which is opposed to the exposed portion through separator 124, and electrolytic anions BF4−, PF6− and the like are drawn to the anode side, to acidify the peripheral portion thereof, thereby causing deterioration in separator 124. Hence there is a problem of consequently causing not only deterioration in characteristic but also occurrence of gas generation and an increase in inner pressure.
It is possible to solve such a problem all at once by forming polarizable electrode layer 126 on the lead wire and exposed portion of current collector 125 after connection of the lead wire to each electrode, but adding such a process forces a large burden in terms of cost.
[Patent Document 1] Unexamined Japanese Patent Publication No. H08-83738
[Patent Document 2] Unexamined Japanese Patent Publication No. H10-270293